Telecommunication system



June 19, I951 M. DEN HERTOG TELECOMMUNICATION SYSTEM 3 Sheets-Sheet 1 Filed Feb. 27, 1943 lNVENTOR MART/N05 051v HERTUG ATTORNEY June 19, 1951 M. DEN HERTQG' 7 2,557,638

TELECOMMUNICATION SYSTEM Filed Feb. 27, 1948 v 3 Sheets-Sheet 2 MART/NUS DEN HERTOG ATTORNEY June 19, 1951 M. DEN HERTOG I 2,557,638 TELECOMMUNICATION svsm Filed Feb. 27, 1948 s sneets-sheet s INVENTOR MA RT/NUS 0m HE/Pr0c fazaw/wel ATTORNEY Patented June 19, 1951 TELECOMMUNICATION SYSTEM Martinus den Hertog, Antwerp, Belgium, assignor to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application February 27, 1948, Serial No. 11,403 In the Netherlands January 7, 1947 7 Claims.

The present invention relates to automatic or semi-automatic telephone systems and other telecommunication systems, in which power-driven switches are used for the establishment of con nections or for other signalling purposes, and more particularly to improved methods for controlling the positioning of these switches accurately in predetermined stopping positions, resulting in a quick and reliable operation.

The principal object of the invention is to provide a double test circuit arrangement for controlling the operation of selector switches of a telecommunication system in which when the stopping operation has once been initiated it will continue for a predetermined time, sufilcient to permit the centering of the wipers of the switch even though they have overshot the desired stopping position, thus permitting the attainment of higher speed and more accurate operation than with apparatus heretofore used.

Another object of the invention is to provide an arrangement which permits a bank construction of a selector switch having terminals with which contact is made during a relatively small part only of the total time of the step.

In order to render the purpose of the invention clear, it should be observed first that with the power-driven apparatus actually used, three distinct methods are employed with a view to obtaining proper stopping in a correctly centered position.

1. BY TIMING The first method is that used with the well known rotary selectors, of which the stopping is based on the time required to perform certain electro-mechanical operations (e. g. operation of test relay, release of loop control relay, release of clutch magnet).

It is assumed that the time required for these operations, which commence at the moment the test brush enters into contact with the test terminal of a free outlet, will cause the brush carriage to stop in a position in which all brushes are in contact with, and have developed a sufficient pressure on, the arc terminals of the outlet tested, with due regard to all causes of variation that may occur. These variations are of principally two kinds, viz. mechanical (accuracy with which terminals and brushes are lined up, variation of length of contact closure due to manufacturing limits) and electrical (battery potential, electrical adjustment of relays and clutch magnet, shaft speeds, line conditions).

In some selectors of this type a centering contact has been provided on the brush carriage, for

the purpose of preventing the de-energisation of the clutch magnet during the interval when moving from each terminal to the next, but it is extremely diflicult to obtain the close manufacturing limits to ensure its effectiveness under all conditions.

2. BY REDUCED SIZE or TEST BRUSH The second method is that applied in the well known rotary line finders of the gear-driven type, of which the test brush is reduced to such a size that the closure of this brush guarantees the closed condition of all other brushes.

A general object against this arrangement is that it has been found impossible to use the test brush for other functions after testing, because its closure is not guaranteed after the finder has been stopped.

3. BY MECHANICAL CENTERING Certain power-driven switches have a mechanical centering arrangement which, to a limited extent, forces the brush carriage to occupy a centered position with respect to the terminals on which the switch has tested. This mechanical arrangement can become efiective only, however, if the switch is arrested early enough to prevent the test brush from overshooting the test terminal of the outlet tested, because once the test circuit is opened, the test relay which operated therein at the moment of testing, releases again and causes the switch to continue its motion.

In practice it has been found that neither of the two methods referred to first may definitely prevent cases of late stopping of the switch, resulting in the brush carriage being stopped in a position in which one or more of its brushes have passed over the contact on which it should have stopped.

Such cases of late stopping may occur when a selector meets an outlet which becomes free at a moment the brushes have already partially completed their travel over its terminals, so that the test relay does not start to operate right from the moment the test brush meets the test terminal but some milliseconds later,

This condition may, with everything normal and all adjustment perfect, bring the brushes right to the end of the terminal, or even past it. This explains why rotary selectors are sometimes found with their test brush still closed at the far end of the terminal and their A or B brushes, or both, open.

It explains also Why finders of the power-driven type are found stopped in a position in which When providing switches with mechanical centering and using a test arrangement in accordance with this invention, these faults can be completely eliminated.

A second improvement in the operation which may be obtained by the arrangement according to the invention is that when the test circuit controlling the hunting of selectors or line finders contains a cold cathode tube, mechanical centering may be used to advantage in order to obtain a higher speed of hunting than is possible otherwise.

A third advantage that may be gained from the arrangement according to the invention is that it permits a bank construction in which the terminals close during a relativel small part of the total time of a step.

According to one feature of the invention, power-driven switches are used with mechanical centering, preferably by retractile spring action of the movable parts in the stopping positions, in combination with a test arrangement which, upon having been caused to function as a result of the switch arriving in a position in which it should be made to stop, is rendered henceforth inert to the electrical condition prevailing on the test conductor, until after a lapse of time sufficient to cause the switch to be stopped in a centered position, upon which a second test is made to ascertain that stopping has happened in a correct position.

According to a second feature of the invention one of the elements which constitute the test arrangement is a gas filled tube, which ionizes when the electrical condition on the test conductor indicates that the switch has arrived in a position in which it should stop, and which, from the moment it has ionized, is unafiected by the electrical condition on the test conductor, and thereby causes the continuation of all functions required to stop the switch, also if this should have moved momentarily or definitely beyond the position in which it should stop and thereby caused a transitory opening of the test circuit or the connection of a test potential which may be present in the next stopping position of the switch.

According to another feature of the invention the hunting speed of a switch may be increased to the value whereby the time required for stopping exceeds the time during which contact is established with the test contact of the position tested.

According to another feature of the invention the hunting speed of a switch of the type which is mechanically centered by the action of a retractile spring in a direction opposite to the direction of hunting, is to be determined so that the minimum time required for restraining the switch from further forward motion will, when calculated from the time contact is established with the test contact of a position, bring the switch in the minimum forward position required to permit the mechanical centering device to retract the movable parts to the correct stopping position, whereas the maximum time required to restrain the switch from moving forward will, when calculated from the time referred to, bring the switch in the position before the next centering position, thereby permitting the movable parts to be retracted to the position in which test potential was encountered.

The invention makes use of switches of any suitable design permitting mechanical centering of the moving parts in the stopping positions, but preferably of the kind as described in Dutch Patent No. 61,716 of September 15, 1948, to Martinus den I-Iertog, and Belgian Patent No. 464,219 of April 1946, to Martinus den Hertog,

The test arrangement used for controlling the hunting of the switch may be of the kind as described in U. S. application Serial No. 765,022, filed July 31, 1947, by L. Cabes, and U. S. application Serial No. 726,359, filed February 4, 1947, by Martinus den Hertog, or may be of the kind as described in U. S. Patent No. 2,424,585 issued to S. Simon et al. or U. S. application Serial No. 765,021, filed July 31, 1947, by L. Cabes, or U. S. Patent application No. 485,327, filed May 6, 1943, by Henry F. Herbig. Any other suitable arrangement may be used which will be inert to the electrical condition prevailing on the test conductor immediately after it has functioned.

The principle of making a second test after stopping the switch has been disclosed in U. S. Serial No. 473,278, filed Jan. 22, 1943, now Patent No. 2,452,578.

The invention will be more clearly understood by referring to the drawing, in which Figs. 1a and 1b, placed side by side in the order mentioned, represent circuit details of a selector switch and of a register circuit controlling the setting thereof. Fig. 2 is a diagram showing relations between hunting speed and stopping time. Fig. 3 represents an alternative testing arrangement in accordance with the invention.

One of the purposes of the invention is to eliminate the possibility of incorrect stopping of a switch due to so called late testing, occurring when a selector meets an outlet which becomes free at a moment the brushes have already partially completed their travel over its terminals.

Referring to the drawing, it will now be explained how this is accomplished according to one embodiment of the invention:

The drawing Fig. 1 shows at its left-hand side a part of a register circuit controlling a number of consecutive stages of selectors of which three have been shown, viz.; a 1st group selector, a 2nd group selector and another selector connected in the arc of the 2nd group selector. The 1st group selector has not been shown completely, but is shown only in order to indicate how connections are extended via this selector from the register to the 2nd group selector of which the operation will be described. The selector circuit connected in the bank of the 2nd group selector has also been indicated partially only.

The register employs a method of controlling the setting of the selectors known from U. S. application Serial No. 473,278, filed January 22, 1943, by Jakob Kruithof et al., which is based on the comparison of sources of electric current used to characterise different groups of outlets in the selector arc. The particular method, by means of which this comparison takes place, has been disclosed in U. S. application Serial No. 765,021, filed July 31, 1947, by L. Cables, and comprises the two transformers PTI and PT2,

the two cold cathode tubes SV1 and SV2, the anode relay Wpr and a number of resistances and condensers associated therewith.

The selectors shown are supposed to be of the type disclosed in Dutch Patent No. 61,716 of September 15, 1948.

When a call is made a register and a 1st group selector are seized in the usual manner whereupon a 2nd group selector is seized by the 1st group selector. Fig. l of the drawings shows the necessary portions of these parts after the seizure of the 2nd group selector. The 2nd group selector is now set by the register. The digit controlling the settin will have been stored on a number of storin relays Ql-Ifl, not shown on the drawing but indicated in the upper left corner of Fig. 1, and these will connect via their contacts a source of reference current to the primary winding of transformer PT1 via a 200 w. resistance. In the well known way this current will cause the ionisation of the cold cathode tube SVl which will supply a rectified AC-component to the system of condensers and resistances connected to its anode, so that these condensers will be charged to a potential sufficiently high to hold the cold cathode tube 8V2, to the control electrode of which they are also connected, in a deionised condition.

When the 2nd group selector must be started, a circuit is closed by means of a starter contact st for the operation of relay Hpr at the register via break contacts of relays For and Ter, and Hm connects ground to the anode relay Wpr, so that this may operate if SV2 should ionise. At

the same time Hpr connects ground from break contact of relay Wpr and via break contacts of For and Gsr, the 0 brush of the 1st group selector and via a break contact of relay Br and a make contact of relay Ar at the 2nd group selector, to the power magnet P of this selector. It should be noted that relay Ar in the 2nd group selector was already operated in advance at the moment the 2nd group selector was seized by the 1st group selector by ground supplied to it via the a brush of the 1st group selector and through the retardation coil FS at the register in an obvious circuit. The 2nd group selector now rotates until its e brush meets the test terminal of an outlet to which a source of signalling current is connected of which the electrical r characteristics are related in a predetermined manner to those of the source of reference current which is connected at the register to transformer P'Ii, thereby indicating that this outlet is one of the wanted group. When this occurs,

the signalling current will be connected via brush 6 of the 2nd group selector, brush b of the 1st group selector, through the primary winding of transformer PT2 and next via brush a of the 1st group selector to a ground supplied at the 2nd group from a make contact of the power magnet P, and a, make contact of relay Ar. At this instant the cold cathode tube SV1 extinguishes in the well known manner, so that the set of condensers connected to its anode will discharge and thereby cause the tube SV2 to ionise and relay Wpr to operate. This opens the circuit for the power magnet P of the 2nd group selector.

Diiferent cases may now occur:

a. Normal operation The power magnet P of the 2nd group selector will be de-energised after a certain lapse of time from the moment the test brush e enters into contact with the test terminal of the wanted outlet. This time comprises the de-ionisation time of tube SVl, the discharging time of the con-' densers connected to its anode to a value sufficient to provoke the ionisation of SV2, the ionisation time of SVz, the operating time of the anode relay Wpr, the releasing time of the power magnet P of the 2nd group selector and the time to brake the movable parts of the selector so that they come to a standstill. We will call the minimum time required for these operations at, and the maximum time 11. These times have been represented in the diagram Fig. 2 by lines III and IV. Line II in this diagram indicates diagrammatically the relation between the time of closure and opening of two successive selector contacts, the line ab or ab' indicates thereby the time during which each contact is closed, whereas the interval ba or ba" indicates the opening in between each two successive contacts.

The points 0 and c on the line a-b or a'b indicates the centering position in which the brushes should be made to stop ultimately when the selector seizes the corresponding outlet.

The line I in the diagram indicates the corresponding condition of the toothed selector which is provided on the selector frame and over which a pawl is moving durin the rotation of the selector, which pawl is mounted rotatably on the rotor of the selector.

From the diagram it may now be seen that when the power magnet of the selector has deenergised, the position of the brushes with respect to the are terminals may be anywhere between the points d and e, depending on the time required to perform the operations described above. As has been described in Dutch Patent No. 61,716 of September 15, 1948, the brush carriage of the selector is now, under the influence of a, rectractile spring, caused to perform a backward movement until the pawl that runs over the toothed sector meets the steep side of the tooth. When this has happened the brushes will be located on the terminals in the point indicated by c and, therefore, they will have moved in a backward direction to point 0 from any position in which the forward movement was initially arrested and which may be anywhere between the points d and c.

It may be observed that the minimum operating time a: should be so determined that it always exceeds the time ac because if this requirement would not be met, the selector would not be arrested on the terminal on which it had tested, but would slip back to the preceding terminal.

b. Case 0 late testing Assuming now a case where an outlet is still busy at the moment a selector arrives on it, but becomes free during the time the brushes travel over its contacts. Referring to Fig. 2, the point 7 indicates the position of the brushes at the moment it is assumed the outlet becomes free and, therefore, the time j-b is available for causing the operation of the test arrangement. This time may be quite sufiicient, because for ionising the tube SVz a very short space of time is required only, and the connection of a test potential for approximately 1 or 2 milliseconds will suflice. In lines V and VI, the stopping times have been indicated by :t for the minimum time and y for the maximum time respectively. It will be seen that in this case stopping will still be 7 guaranteed before the centering position 0 of the next terminal is reached and accordingly the brush carriage, upon having been arrested will be retracted from any point between 9 and h to point e on the terminal just liberated and tested again.

It will be observed that the test lead was either opened (e. g. for the time 17-9) or it might have again become closed on the next terminal for the time a.-h. Neither the opening nor the connection of another test potential which may be present on the test terminal of the next outlet, may aliect however, the functioning of the circuit as described before, because even if by said opening or by the reconnection of another test potential, the tube SV1 should ionise again, it will be unable to extinguish the tube SVz, because this, once ionised, will maintain its ionisation independent of the condition on its control electrode. Therefore, the anode relay Wpr will, in any case, remain operated and cause the continuation of all operations required to stop and center the selector in the correct position.

After the operations described above, the register circuit now proceeds to make two consecutive tests on the outlet tested, in order to make sure that the selector has stopped on a right outlet. The first test is a test for the presence of battery potential in the outlet, which serves at the same time to take care of possible cases of simultaneous seizure (double test) The second test is a repetition of the test for the presence oi the characteristic signalling potential on the test lead of the outlet, in order to make sure that the outlet is one of the wanted group and that the selector has not stopped accidentally on an outlet of a diiierent group. It will be evident that these tests may commence only after the brushes of the selector have been positioned in the correct centering position. This is guaranteed by the fact that before the DC-test takes place, the relay Pcr at the register has to operate. This relay provides sunicient time for the stopping and centering of the selector to be completed. It operates from the make contacts of Wm via break contacts of Gsr and Tar, and when energised, closes a circuit for the operation of the double test relay Dir via brush d of the let group selector and make contact of relay fir brush 0! at the 2nd group selector to test potential found in the outlet tested, via a break contact of relay A1" of that outlet and 24920. If this test is successful, Dtr operates and closes a circuit through the winding of Dpr 311:- its own low resistance winding in parallel rendering the tested outlet busy for other calls. Dpr operates also and prepares a circuit for the operation of C-sr which will be complete when both Hpr and Wm have released. This is caused by relay For which opens the circuit of Hp," and this in turn releases Wpr and extinguishes the valve SVz.

After the operation of Gsr, the second test on the AC-signalling potential takes place, because now the circuit for I-Ipr is reclosed at a contact of Gsr, viaa break of Ter, and this relay again prepares the same test circuit as was 8.1 played during the hunting of the selector. If the selector stands on a correct outlet, Wpr operates again preventing thereby the selector from moving from this outlet because it will open the circuit for the power magnet of the brush carriage at its break contact before this power magnet can energlse and further W 221' will energise relay Okr, which switches the register to a different condiwith its operating circuit, thereby tion in which it will control the next selection and which will also cause the operation of Br of the 2nd group selector in series with AT by openthe ground via the a brush of the 1st group selector. This brings the 2nd group selector in the through-switched condition.

From the description of the operation given above it may be seen that on one hand the arrangement described will guarantee that a hunting selector will be properly centered on the terminals of the wanted outlet, even if the brushes had nearly passed these terminals at the moment the outlet became free; on the other hand precautions are taken that no wrong or busy outlet can be engaged on which the switch is forced to stop by an accidental operation of the test arrangement, e. g. by a momentary presence of a free test potential.

A second purpose of the invention is to provide an improved arrangement by which higher hunting speeds of the switches may be obtained than by other known arrangements.

That this is so, may be seen from the following considerations The cold cathode tube is caused to fire in a very short space of time (approximately 1 millisecond) after the test brush has entered into contact with a terminal on which the wanted test potential is present, and will not extinguish, even when this test circuit is interrupted immediately thereafter. The firing of the tube causes the stopping of the switch by the consecutive operation of the anode relay and the release of the clutch magnet.

Without mechanical centering the speed of hunting has to be limited to the value whereby the total maximum time required to stop the switch is less than the minimum time of contact closure, as only in this way it may be ensured that the switch is stopped before the switch passes the contact.

With mechanical centering the speed of hunting may be chosen to be considerably higher, as the only requirement is that the switch is stopped before it reaches the centering position of the next terminal. In this case it will be reset to the centering position of the terminal On which the tube fired. This evidently means that the brush may temporarily have passed this terminal, or even come into contact with the next terminal. fact that the test circuit is momentarily opened by the brush leaving the terminal, or that the test brush connects momentarily a different potential which may be present on the next terminal, is of no importance, since the cold cathode tube, once fired, cannot be affected any more by any subsequent condition on the test wire and causes the continuation of the further operations required to stop the switch. The sectest to be made on the outlet tested should be made after an interval suificient to permit the mechanical centering device to complete its action to restore the brush carriage to the centering position. As this restoration happens very rapidly, the interval provided by the operating time of an ordinary relay is sufficient for this purpose.

Assuming now that when the hunting speed has been chosen so high that in a case of normal testing the maximum time required for arresting the brush carriage in its most forward position, would bring the brushes very near to the centering position or the next terminal, it should be investigated what happens in a case of late" testing. In such a case it may evidently occur that the brushes cannot be arrested in time beenergisation of H 9 fore the centering position of the next terminal and consequently the selector will be stopped thereon. Two possibilities now exist:

If the next terminal belongs to the same group as the terminal on which the selector tested ini tially, and is free, it will be engaged by the register in the normal way, because the register will proceed with the DC-test and the second AC-test in the manner as described before, and both these testswill be successful. If, however, the next terminal is either busy or belongs to a difierent group of outlets, the register will cause the selector to resume its hunting action. In this case, therefore, the fact of late testing does not cause any trouble, but will simply slightly delay the seizure of a free outlet. The manner in which the register causes the selector to resume hunting is as follows:

CASE OF NEXT TERMINAL BUSY At the moment Pcr operates consequent upon the first operation of Wpr, relay Dtr will fail to energise because it does not find free test potential in the outlet on which the selector stopped. In consequence thereof, a circuit will be closed for the operation of relay Dmr via break contacts of Dtr and Em and the make contact of P01. Dmr opens the circuit from the d brush of the 1st group selector to the double test relay Dtr and instead connects an arrangement suit able for controlling the hunting of the selector and which in this particular case is a static switch Sw such as described in U. S. application Serial No. 765,022, filed July 31, 1947, by L. Cabes. Relay Fir represented on the drawing is the anode relay of the cold cathode tube of the static switch. At the same time Dmr connects ground, under the control of a break contact of Ftr, to the 0 brush of the 1st group selector and thereby operates the power magnet P of the 2nd group selector which, therefore, resumes hunting until it meets a free outlet in which battery potential is present on the c2 terminal. At this instant the static switch functions, causing Ftr to operate and the power magnet to de-energise. Fir on its make contact and via a make contact of Dmr, operates Ter with a delay suificient to ensure that he switch has stopped in its centered position. Ter opens the circuit for For which releases and thereby also causes Dmr to release. This disables the static switch, so that Fir releases and further causes Ter to de-energise. This closes the circuit for the re-cperation of relay Em and now the circuit is again in the initial condition in which it controls the huntingof the selector for an outlet in a particular group under the control of the digit dialled. If the selector should happen to have stopped on an outlet belonging to the wanted group, the relay Wpr will operate instantly before the power magnet can again be energised, if, however, the outlet belongs to a different group, the circuit for the power magnet of the group selector will be re-closed at a make contact of Hpr to ground at a break of Wm.

CASE OF NEXT TERMINAL BELONGING TO A DIF- FERENT GROUP OF OUTLETS In. case the next terminal is free but belongs to a different group of outlets, relays Dtr and Dpr will operate in a normal way, but upon the second relay Wpr will fail to energise. The circuit for the power magnet of the group selector is not immediately closed in this case, because it is kept open at a break contact of Gsr, but a circuit will be closed over make contacts of Hpr and Gsr via a break contact of Per,

19 for the operation of Ter, relay Per releasing slightly slowly owing to its holding circuit having been opened by the 2nd operation of Hpr, because this holding circuit is controlled via breaks of I-Ipr and Gsr which are now both en-.,

ergised.

Ter, by operating, locks itself via a make of Gsr and causes the de-energisation of Hpr. Further Ter opens the circuit for Dior and Dtr which release and in turn cause the release of Gsr. Gsr causes the de-energisation of Ter, which is the last relay energised and which re-closes the circuit for Iipr, so that the operation may now recommence from the beginning.

It will, therefore, be seen that also in the case now under consideration, the switch can in no way be forced to engage a wrong or busy outlet, on which it is forced to stop as a result of late testing.

The third purpose of the invention is to provide an improved arrangement which permits a bank construction in which the terminals close during a relatively small part of the total time of a step only.

With existing power-driven switches it has always been attempted to design the arc terminals and the brushes in such a manner that maximum closure is obtained and a minimum opening is provided between terminals, consistent with the requirement that no overlapping between adjacent terminals should take place. Thus, for the line brushes of the gear-driven finder the closing time was fixed at 90%i2% of a step and for the brushes of the rotary selector the closing time is between the limits of and 98% of a step.

This was done on account of the desire to provide the maximum possibility for the switch to stop on the terminals, and so as to be able to increase the speed of hunting to the maximum possible.

When mechanical centering is provided, combined with a test arrangement containing a gasfilled tube according to the invention, the brush carriage comes to rest in the same position on each set of terminals always, and thereby the possibility is provided to reduce the length of the closure to the amount needed to cover only the possible variations of the alignment of brushes and terminals.

It should be observed that the reduction of the closure of the test terminal has no adverse alTect on the capability of the switch to stop at high speed, provided the test circuit contains a gas-filled tube, which is cap-able of firing and then is bound to bring the switch to a stand still, if the test potential is encountered for the short space of time of approximately 1 millisecond only.

The facility provided according to the invention to reduce the length of closure of the arc terminals opens up the possibility of employing designs of arc terminals and contacting member hitherto impossible or impracticable.

For example, when using a roller type or contact, the arc may be so shaped that the roller passes over isolated metal pieces or dummy terminals located in between each two adjacent arc terminals, so as to avoid the possibility of the roller establishing a momentary short-circuit between these are terminals, or other similar design may be employed resulting in the same effect.

embodiment of the invention, in which no gas filled tubes, but relays exclusively are used in Fig. 3 of the drawings shows an alternative the test arrangement. This may be used to advantage in cases where the test arrangement is not provided in a circuit provided in common for a number of switches, but where it has to be provided with every switch individually.

The operation according to Fig. 3 is as follows: The drawing represents one of a number of switches of which the arc contacts are multipled, and of this switch four brushes 0., b, c and d and the power magnet P are shown. It is assumed that upon the arrival of a call one or more switches will hunt for a test terminal (1, to which a free test potential is connected by the closure of a contact S1 and further, that simultaneously the circuit for one or more of the power magnets is closed via an individual contact S2 provided per switch on the starting relay. In such a case,

'the power magnet P will operat to ground at a break contact T1 of the test relay Tr and cause the brushes to sweep over the arc terminals until the circuit for the test relay is closed at the moinent the test brush encounters free potential. Relay Tr then operates via its 300 ohms winding and at its breakcontact opens the circuit for power magnet P. quick relay, which may operate in say 1 millisecond, and then closes a locking circuit via its 60 ohms winding to battery via the winding of Hr and a 400 ohms resistance in parallel. Relay Tr once operated, will therefore hold independent of its operating circuit, so that the test brush, prior to the moment the brushes are brought in a centered position, may open momentarily or even contact with the next test contact in the are for a short instant, without thereby affecting the concondition of relay Tr.

After a short time, suflicient to complete the centering action of the switch, relay Hr operates. The contacts 01" this relay are so disposed that make contact H1 closes before the break contact of change over H5 opens. As a consequence, ground will be closed through the 7 ohms winding of Tr, in parallel with the 309 ohms winding, before the 60 ohms winding is short circuited at make contact of H5. When closing the ground at H1 and assuming the brushes have been centered properly on the outlet tested, this outlet will be rendered busy immediately to other hunting line finders, and Tr will be able to hold on its 309 ohms and 7 ohms winding in parallel, also after the 60 ohms holding winding is short circuited by make contact H5. In this case Hr will remain operated to ground at make contact T1 via its own make contact H5, and at contacts H2, H3 and H4 it will complete the circuits for brushes a, b and c of the switch.

Assuming however, that, upon the centering function having been completed, no free potential exists on the test terminal on which brush d stands, then upon the closure of make contact H5, the 60 ohms winding of Tr will be short-circuited, and thereby this relay is de-energized, as it is unable to hold on its two other windings. In consequence, when mak contact T1 opens, relay Hr, which is a quick-to-release relay will also be de-energized, and releases quickly, whereby the hunting condition is restored, the circuit for P being re-connected at break of T1.

It will be seen that the principle of this arrangement is, that immediately upon the operation of the test relay Tr when encountering free potential, this relay is locked locally and thereby rendered independent of the condition on the test wire. This condition lasts for a time suflicient tocomplete the mechanical centering of the switch,

The relay Tr is an extremely upon which Hr operates, which renders the looking winding of Tr inactive, and causes this to check a second time for the existence of free potential. If this should be absent, for example because of late testing, which caused the switch when running at high speed to overshoot to the next terminal, relay Tr and consequently Hr release again and thereby restore the circuit to the hunting condition. The same happens when in case of double test, one finder succeeds in busying the test terminal before another, as then the relay Hr oi the last mentioned finder, by operating, will also cause the associated test relay to de-energise.

In this case, contacts H2, H3 and H4 may become momentarily closed, but as both relays Tr and Er will release extremely fast, this time is insignificant, and the closure of these contacts has no further consequences.

It will be clear that by virtue of the fact that the test relay is made independent of the condition on the test conductor, immediately upon the closure Of its make contact, all advantages claimed for the first embodiment of the invention, arealso provided with the second. In order that this desired condition is created, not only has the operating time of the test relay to be very small, but also the time required to build up the current in its holding winding shall be made very short. This is obtained according to Fig. 3 by providing a resistance in parallel with the winding of I-Ir and by dimensioning the holding winding and this resistance such that the relay can hold via this resistance alone, disregarding the presence of the winding of relay Hr. The double purpose is thereby obtained that not only does the impedance of the winding of Hr not have an adverse effect on the time required to build up the holding current in the 60 ohms winding of Tr, but also the relay Hr is thereby rendered slightly slow in operation, which is required in order to permit the switch to complete its centering action before Hr operates. After Hr has energised, the parallel connection of the resistance to the winding of I-Ir is opened, in order that this relay may release rapidly in case of an unsuccessful test, as explained hereabove.

This invention is not limited to the shown embodiments so that several modifications are possible within the scope of the invention.

What is claimed:

1. In a telecommunication system having a selector switch with a plurality of terminals arranged in sets, each set including a test terminal, a plurality of wipers movably mounted and arranged for respective engagement of the terminals of a set, and driving means {or said wipers for causing successive engagement of said sets of terminals, the combination of means for applying a predetermined electrical condition to the test terminal of a set of terminals on which it is desired to stop said wipers, means controlled by the initiation of a call for energizing said driving means, for de-energiz-ing said driving means, and means connected to the wiper which engages said test terminals and which is responsive to said predetermined electrical condition for initiating the operation of said de-energizing means, said initiating means including means for continuing the operation of said deenergizing means, once it is initiated, for a predetermined time regardless of other electrical- .conditions which may be applied to said initiating means during said time.

2. In a telecommunication system, the combination as claimed in claim 1, in which the means for continuing the operation of the deenergizing means comprises a gas-filled tube and means for ionizing said tube when the predetermined electrical condition appears on the test terminal, said tube being of the type which, from the moment it is ionized is inactive to the electrical conditions on the test terminal and thereby permits continuation of all functions required to stop and center the switch.

3. In a telecommunication system the combination, as claimed in claim 1, in which the responsive means comprises two relays having contacts operated thereby, a circuit connected to a contact of the first of said relays for causing the responsive means to de-energize the driving means when said first relay operates, said first relay having a winding connected to the wiper engaging the test terminals, whereby the first relay energizes when the wiper arrives at a terminal on which said predetermined electrical condition is present, said first relay having a holding circuit closed through one of its own windings and a back contact of said second relay and also including the operating circuit for said second relay, said second relay having a predetermined operating time delay, so that it oper-- ates after a time delay sufiicient for the switch to complete a centering action, the operation of that at the lowest speed of hunting, the minimum time required for restraining the switch from further forward motion upon testing will, when calculated from the moment contact is established with the test terminal of a set of terminals, bring the switch beyond the centering position by at least the minimum distance required to permit the wipers to be retracted to the correct stopping position, whereas at the highest speed of hunting, the maximum time required for said purpose will, when calculated from the moment referred to, bring the switch into a position at the minimum distance before the next centering position which is required to permit the retraction of the wipers to the position in which the test potential was encountered.

6. In a telecommunication system, the combination as claimed in claim- 1, further comprising means responsive to a difierent electrical condition, means operated by said de-energizing means for connecting said last-mentioned responsive means to another one of said wipers a predetermined time after said de-energizing means operates, means controlled by the operation of said last-mentioned responsive means for sensitizing said first-mentioned responsive means for a second operation, and means controlled by the second operation of said first-mensaid second relay acting to disable the holding winding of the first relay and thereby causing said first relay to check by means of said windin connected to said test terminal for the presence of said predetermined electrical condition on said test terminal, whereby, depending on the presence or absence of this condition after the centering has taken place, the first and second relays are caused either to hold or to release consecutively.

4. In a telecommunication system, the combination, as claimed in claim 1, in which the components are so chosen that the time required for stopping the switch may exceed the time during which contact is established with the test terminal of the set of terminals tested.

5. In a telecommunication system, the combination, as claimed in claim 1, in which the hunting speed of the switch in a direction opposite to the direction of hunting is adjusted so tioned responsive means in response to said predetermined electrical condition on said test terminal for completing connections to said wipers.

7. In a telecommunication system, the combination as claimed in claim 6 in which means is provided under control of the means for making the second test for connecting said driving means again to said switch upon the second test indicating that the switch is not on the desired position and restoring the responsive means to normal again whereby hunting of the switch is resumed.

MARTINUS DEN HERTOG.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,592,323 Aldendorfi July 13, 1926 1,799,485 Stehlik Apr. 7, 1931 2,068,726 Wochinger Jan. 26, 1937 2,419,540 Deakin Apr. 29, 1947 

